This invention relates to a speech channel or line switching network suitable for use in combination with an electronic key telephone set, and more particularly a speech channel switching network suitable for use in a key telephone system in which the speech channel is controlled directly by the supply of power from a telephone exchange of a telephone PBX or a telephone office.
Heretofore, in electronic apparatus having a simple exchanging capability such as an electronic key telephone set, PNPN transistor or diode switching elements or C-MOS transistor analogue electronic switching elements have been used as the speech channel switch.
Examples of the apparatus utilizing such electronic elements are shown in FIGS. 2 and 3, in which the former illustrates the construction of a prior art speech channel switching circuit utilizing such high loss electronic switching element as a C-MOS switching transistor, while the latter illustrates the construction of a prior art speech channel utilizing such electronic switching element as a PNPN transistor or diode switching element capable of ON.OFF controlling the speech currents.
To help easy understanding of the reason of using such circuit constructions, the construction of an earlier key telephone set not utilizing any electronic switching element will firstly be described with reference to FIG. 1 of the accompanying drawing which diagrammatically illustrates principal elements of the prior art key telephone set. As shown, it comprises a common control equipment or key service unit 1, input central central office lines 2a, a push button or key telephone set 3, a telephone set circuit 4, local lines 2b and central office line selection switches 5a, 5b, 5c and 5d. The input central office lines 2a to the common control equipment 1 and the telephone set circuit 4 are interconnected by central office line selection switches 5a through 5d via local lines 2b. For this reason, the DC resistance (loop resistance) of the common control equipment 1 as seen from the input central office lines 2a is equal to the sum of the resistance of a cable (that is the local lines 2b) between the common control equipment 1 and the key telephone set 3, and the internal resistance of the telephone circuit 4 (the total sum of the resistance of a transmitter, the resistance of a transformer winding, and an equivalent resistance caused by the voltage drop in a polarity reverse preventing circuit). In Japan, the loop resistance is prescribed by a regulation to be less than 220 ohms, because a maximum value of the line resistance to a telephone office is prescribed to be 1500 ohms, and even for a subscriber under such adverse condition, it is necessary for the telephone exchange in the telephone office to assure current (response current) necessary to drive a line relay provided for detecting whether or not the subscriber has hooked off its handset for talking.
For this reason, it has been impossible to use a terminal device (including a terminal device having a simple exchange capability) connected to a telephone line (a subscribers line) the internal DC resistance of which cannot be reduced to be less than 220 ohms. This is also true for the aforementioned electronic key telephone system utilizing electronic switching elements.
Usually, the internal resistance of the telephone set of a key type telephone set such as that disclosed, for example, in a paper entitled "Development of the Pushbutton Telephone Set", Electrical Communication Laboratories Technical Journal, Vol. 17, No. 11, page 2539, 1968, is equal to the sum of 90 ohms (the transmitter resistance plus the resistance of the transformer winding) and an equivalent resistance of 70 ohms caused by the voltage drop in a bridge rectifier provided for supplying current of a definite polarity to a circuit (MFO) which generates a key dial multi-frequency signal. More particularly, the voltage drop caused by two diodes selected from 4 diodes comprising a bridge rectifying circuit amounts to 1.4 V, and the current flowing through a line having a resistance of 1500 ohms is 20 mA so that equivalent resistance equals 1,4 V/0,02 A=70 ohms. When the loop resistance of 40 ohms is added to (90+70) ohms, the total resistance of the common control equipment 1 as seen from the input central office lines 2a becomes 200 ohms.
Thus, the key telephone set shown in FIG. 1, which does not utilize any electronic switching element, satisfies the regulation that the loop resistance should be less than 220 ohms.
However, when the central office line selection switches 5a through 5d are substituted by electronic switches, it becomes impossible to satisfy the regulation. For example, a C-MOS transistor switch has a resistance of about 100 .OMEGA. when it is in an ON or conductive state and even a modern one has a resistance of about 80 ohms so that it can not satisfy the regulation requiring less than 220 ohms. Generally, a C-MOS transistor can not be used in a speech channel switching circuit because its breakdown voltage is low, e.g. about 10 V, its current capacity is also small, e.g. about 20 mA and its ON resistance is high noted above. If it is necessary to use C-MOS transistors as the switching elements, the circuit should be constructed as shown in FIG. 2. In the circuit construction shown in FIG. 2, the central office line selection switches 5a through 5d are gathered in the common control equipment 1. In FIG. 2, the circuit elements corresponding to those shown in FIG. 1 are designated by the same reference characters, and numeral 6a designates a central office line transformer, 6b a local line transformer, 7a and 7b C-MOS transistor switching elements, and E a power source for supplying speech current to the key telephone set 3. With this construction, for the purpose of setting the internal resistance as seen from the input central office lines 2a to 220 ohms, a central office line transformer 6a is provided and the circuit is designed to satisfy the regulation by setting the resistance of the primary winding of this central office line transformer to be less than 220 ohms. Similary, for the purpose of decreasing the loss of the C-MOS transistor switches 7a and 7b an impedance transformation is effected by the secondary winding of the central office line transformer 6a to decrease the equivalent loss and then the impedance is transformed back to the original value by the secondary winding of the local line transformer 6b, thus matching the impedance with that of the key telephone set 3. Of course, the speech channel shown in FIG. 2 utilizing C-MOS transistors can be modified variously.
This circuit construction, however, has a serious defect that interruption of a commercial power source that drives the electronic key telephone set results in the interruption of the speech.
To prevent such defect, it is necessary to install a non-interruption power source, for example a back-up battery for the electronic key telephone set, but even when modern low power consumption circuit elements are used it is essential to continue supply of speech current to the key telephone set 3 from the back-up battery. Accordingly, in order to operate the telephone set with the battery over a long interval of from several hours to several tens of hours it is necessary to install a battery having a large capacity of the order of from 10 to several hundreds ampere hours, thus increasing cost and volume and making it difficult to maintain the battery always at the charged condition.
Where a PNPN transistor switching element is used having an ON resistance smaller by one order of magnitude than that of a C-MOS transistor, although it is not necessary to transform the impedance with transformers, where the input central office lines 2a are connected to local lines through PNPN transistor switching elements, since the voltage drop at the PN junctions is about 0.9 V, and the equivalent resistance is 45 ohms for a loop current of 20 mA, the sum of these resistances and the internal resistance of 200 ohms of the telephone circuit 4 shown in FIG. 1 exceeds the prescribed value of 220 ohms described above.
In this manner, the circuit construction shown in FIG. 2 can not be used for remotely located subscribers because of a large equivalent resistance caused by the voltage drop at the PN junctions. As a circuit, a circuit construction as shown in FIG. 3 has generally been used, in which elements coresponding to those shown in FIGS. 1 and 2 are designated by the same reference characters. In FIG. 3, PNPN transistor switching elements are designated by a reference numeral 9. The circuit construction shown in FIG. 3, however has the same defects as the circuit shown in FIG. 2 utilizing C-MOS switching transistors.
It has also been proposed to use, as the electronic switching elements, bidirectional switching elements at each one of four paired cross-points as disclosed in U.S. Pat. No. 4,060,699. This system also involves such problems that the relays of a telephone exchange in a telephone office do not operate for remotely located subscribers, that the line lengths of serviceable subscribers are limited and that switching elements having no self-holding action are used.